Modem battery usage consists of an interesting mix of old technology such as the well-known lead-acid batteries and newer technology such as batteries using various polymer combinations as electrolytes. Most automotive batteries, for example, are still lead-acid types, while lighter-weight batteries typically used for smaller devices such as cell phones use different materials such as nickel-cadmium and/or lithium materials.
The drawbacks associated with batteries that use acids or alkalis as electrolytes have been repeatedly characterized in the relevant literature. These drawbacks include the health, safety, and environmental concerns associated with highly corrosive acid and alkali electrolytes, the disadvantages of using heavier, environmentally-unfriendly metals such as lead for electrodes, and the relatively high weight-to-power ratios of such batteries. While these types of batteries continue to be widely employed, efforts have been made to find alternative electrolyte and electrode materials that are less hazardous. Some of these alternative technologies are described as follows.
In regard to all of the below-described prior art involving the use of ionic liquids, the ionic liquids are used as non-flammable, non-reactive solvent systems. The unique aspect of the battery systems described by this new approach is to create an ionic liquid which is itself a reactive component of the battery system. This introduces a new type of chemistry other than that described in the prior art discussion. In these new systems, the ionic liquid functions not only as the solvent, but as an integral part of the electrochemical processes used to generate electric current from the battery.
Bennion et al. (U.S. Pat. No. 4,234,667) describe and claim a battery comprising a positive electrode, a molten lithium chlorate and/or lithium perchlorate electrolyte, and a negative electrode comprising elementary lithium. While this may exhibit advantages over the use of strong, corrosive acid or alkali electrolytes, the lithium chlorate/perchlorate electrolyte requires an operating temperature range of about 140° C.-160° C. This is not acceptable for many uses, especially for smaller devices such as hearing aids. Also, while lithium is perhaps less environmentally damaging than lead, it is still a concern.
Carlin et al. (U.S. Pat. No. 5,589,291) disclose a battery comprising a lithium metal anode with a lithium salt protective film deposited on the anode, and an electrolyte comprising a molten salt electrolyte (MSE). The MSE can comprise 1-ethyl-3-methylimidazolium and must also contain LiBF4 and water.
Koch et al. (U.S. Pat. No. 5,827,602) disclose a range of ionic liquid cations for a variety of uses. One of the cations is imidazolium as described in Koch et al. at col. 2, line 63, through col. 3, line 7. While Koch et al. disclose that such an ionic liquid can be used as an electrolyte, they specify that such use requires the presence of Li+ in the electrolyte for use in primary or secondary lithium batteries.
Yong et al. (U.S. Pat. No. 7,316,866) disclose a battery cathode produced from a slurry, the slurry comprising a lithium-containing metal composite oxide (e.g., LiCoO2) or a chalcogenide and an ionic liquid. The ionic liquid can be any conventional ionic liquid, including one in which the cation is imidazolium. The slurry is applied to a current collector, with or without a binder, and dried to form the cathode. With respect to the current invention, it is particularly noted that Yong et al. do not disclose an electrolyte comprising an ionic liquid containing imidazolium cation, but instead (see Yong et al., col. 6, II. 41-55) specify an electrolyte solution comprising salts, e.g., Li+, Na+, and/or K+ with an anion.
Kishi et al. (U.S. Pat. No. 7,419,744) disclose and claim a secondary battery having a specified anode, a cathode, and a nonaqueous electrolyte consisting of a lithium salt, an ionic liquid, and a specified ally phosphate. The ionic liquid may comprise an imidazolium cation.
Hollenkamp et al. (U.S. Pat. No. 7,479,353) disclose the use of an ionic liquid as an electrolyte. The cation of the ionic liquid is a pyrrolidinium compound. It is further specified (see claim 1) that the ionic liquid electrolyte must contain lithium ions.